The structure and operation of the present invention will be explained, hereinafter, within the context of improving the function and durability of a non-return valve that is configured for use in a barrel assembly of an injection molding system for the molding of a metal alloy, such as those of Magnesium, in a semi-solid (i.e. thixotropic) state. A detailed description of the construction and operation of several of such injection molding systems is available with reference to U.S. Pat. Nos. 5,040,589 and 6,494,703. Notwithstanding the foregoing, no such limitation is intended to be imposed on the general utility of the non-return valve of the present invention, or its compatibility with other metal alloys (e.g. Aluminum, Zinc, etc.).
The injection molding system, as described hereinbefore, is shown with reference to FIG. 1 and to FIG. 2.
As is commonly known, the injection molding system 10 includes an injection unit 14 and a clamp unit 12 that are coupled together. The function of the injection unit 14 is to process a solid metal feedstock, not shown, into a melt thereof and for the subsequent injection of the melt into a closed and clamped injection mold arranged in fluid communication therewith. The injection mold is shown in an open configuration as comprising complementary mold hot and cold halves 23 and 25. The injection unit 14 further includes an injection unit base 28 slidably supporting an injection assembly 29 mounted thereon. The injection assembly 29 comprises a barrel assembly 38 arranged within a carriage assembly 34, and a drive assembly 36 mounted to the carriage assembly 34, directly behind the barrel assembly 38, for the operation (i.e. rotation and reciprocation) of a screw 56 arranged within the barrel assembly 38. The barrel assembly 38 is shown to be connected to a stationary platen 16 of the clamp unit 12 through the use of carriage cylinders 30 that function to apply carriage forced along the barrel assembly 38 thereby keeping, in operation, a machine nozzle 44 of the barrel assembly 38 engaged in a sprue bushing 55 of the injection mold whilst the melt is being injected into the mold.
The barrel assembly 38, in further detail, is shown to include an elongate cylindrical barrel 40 with an axial cylindrical bore 48A arranged therethrough, the bore configured to cooperate with the screw 56 arranged therein, for processing and transport of the metal feedstock, and as a means for accumulating and subsequently channeling a melt of molding material during injection thereof. The screw 56 includes a helical flight 58 arranged about an elongate cylindrical body portion 59, a rear portion of the screw, not shown, is configured for coupling with the drive assembly 36, and a forward portion of the screw is configured for receiving a non-return valve 60, in accordance with an embodiment of the present invention, with an operative portion thereof arranged in front of a forward mating face 57 of the screw 56. The barrel assembly 38 also includes a barrel head 42 that is positioned intermediate the machine nozzle 44 and a front end of the barrel 40. The barrel head 42 includes a melt passageway 48B arranged therethrough that connects the barrel bore 48A with a complementary melt passageway 48C arranged through the machine nozzle 44. The melt passageway 48B through the barrel head 42 includes an inwardly tapering portion to transition the diameter of the melt passageway to the much narrower melt passageway 48C of the machine nozzle 44. The central bore 48A of the barrel 40 is also shown as including a liner 46 to protect the barrel substrate material from the corrosive properties of the high temperature metal melt. Other portions of the barrel assembly 38 that come into contact with the melt of molding material may also include similar protective linings or coatings. The barrel 40 is further configured for connection with a source of comminuted metal feedstock through a feed throat, not shown, that is located through a top-rear portion of the barrel, not shown. The feed throat directs the feedstock into the bore 48A of the barrel 40, the feedstock is then subsequently processed into molding material by the mechanical working thereof, by the action of the screw 56 in cooperation with the barrel bore 48A, and by controlled heating thereof. The heat is provided by a series of heaters 50, not all of which are shown, that are arranged along a substantial portion of the length of the barrel assembly 38.
The clamp unit 12 is shown to include a clamp base 18 with a stationary platen 16 securely retained to an end thereof, a clamp block 22 slidably connected at an opposite end of the clamp base 18, and a moving platen 20 arranged to translate therebetween on a set of tie bars 32 that otherwise interconnect the stationary platen 16 and clamp block 22. As is commonly known, the clamp unit 12 further includes a means for stroking, not shown, the moving platen 20 with respect to the stationary platen to open and close the injection mold halves 23, 25 arranged therebetween. A clamping means, not shown, is also provided between the clamp block and the moving platen for the provision of a clamping force between the mold halves 23, 25 during the injection of the melt of molding material. The hot half of the injection mold 25 is shown mounted to a face of a stationary platen 16, whereas the complementary cold half of the mold 23 is mounted to an opposing face of the moving platen 20.
In further detail, the injection mold includes at least one molding cavity, not shown, formed in closed cooperation between complementary molding inserts shared between the mold halves 23, 25. In further detail, the mold cold half 23 includes a core plate assembly 24 with at least one core molding insert, not shown, arranged therein. The mold hot half 25 includes a cavity plate assembly 27, with the at least one complementary cavity molding insert arranged therein, mounted to a face of a runner system 26. The runner system 26 provides a means for connecting the melt passageway 48C of the machine nozzle 44 with the at least one molding cavity for the filling thereof. As is commonly known, the runner system 26 may be an offset or multi-drop hot runner, a cold runner, a cold sprue, or any other commonly known melt distribution means. In operation, the core and cavity molding inserts cooperate, in a mold closed and clamped position, to form at least one mold cavity for receiving and shaping the melt of molding material received from the runner system 26.
The molding process generally includes the steps of:    i) establishing an inflow of metal feedstock into the rear end portion of the barrel 40;    ii) working (i.e. shearing) and heating the metal feedstock into a thixotropic melt of molding material by:            a. the operation (i.e. rotation and retraction) of the screw 56 that functions to transport the feedstock/melt, through the cooperation of the screw flights 58 with the axial bore 48A, along the length of the barrel 40, past the non-return valve 60, and into an accumulation region defined in front of the non-return valve 60;        b. heating the feedstock material as it travels along a substantial portion of the barrel assembly 38;            iii) closing and clamping of the injection mold halves 23, 25;    iv) injecting the accumulated melt through the machine nozzle 44 and into the injection mold by a forward translation of the screw 56;    v) optionally filling any remaining voids in the molding cavity by the application of sustained injection pressure (i.e. packing);    vi) opening of the injection mold, once the molded part has solidified through the cooling of the injection mold;    vii) removal of the molded part from the injection mold;    viii) optionally conditioning of the injection mold for a subsequent molding cycle (e.g. application of mold release agent).
The steps of preparing a volume of melt for subsequent injection (i.e. steps i) and ii)) are commonly known as ‘recovery’, whereas the steps of filling and packing of the at least one mold cavity (i.e. steps iv) and v)) are commonly known as ‘injection’.
The non-return valve 60 noted hereinbefore functions to allow the forward transport of melt into the accumulation region at the front of the barrel 40 but otherwise prevents the backflow thereof during the injection of the melt. The proper functioning of the non-return valve 60 relies on a pressure difference between the melt on either side thereof (i.e. higher behind the valve during recovery, and higher in front during injection). The structure and operation of a typical non-return valve, for use in metal injection molding, is described in U.S. Pat. No. 5,680,894.
An example of a typical non-return valve 60 for use in a barrel assembly 38 of an injection molding machine is shown with reference to FIGS. 3A, 3B, & 3C. In further detail, the non-return valve 60 includes: a tip member 62 which is configured to be retained in the forward portion of the screw 56; a ring member 64 which is configured to cooperate with the tip member 62 between a recovery and an injection position defined therealong; and a flange member 66 arranged behind the ring member 64, on tip member 62, to limit the backward travel of the ring member 64 and is further configured to seal with the ring member 64 in the injection position.
The tip member 62 comprises a cylindrical body 63 with a tip flange 81 and a retaining flange 73 arranged on a forward and mid-section, respectively, thereon. Along the length of the tip member 62 are functional portions that include, listed from the rear forward, a screw coupling portion 68, an aligning portion 70, a seat portion 72, an annular flow portion 74, a forward retaining portion 76, a tip circumferential portion 80, a tip tapered portion 82, and a tip planar portion 86.
The screw coupling portion 68 is configured for retaining the tip member 62 in the forward portion of the screw 56, and is provided by forming a helical thread therealong as a part of a threaded coupling.
The aligning portion 70 is configured for axially aligning the tip member 62 with a longitudinal axis of the screw 56, and is provided by a cylindrical mating surface therealong that cooperates with a complementary mating portion 70′ formed in the front portion of the screw 56. The aligning portion 70 is also used for similarly aligning the flange member 66 between a forward mating face 57 of the screw 56 and the seat portion 72.
The seat portion 72 includes an undercut formed at the front of the aligning portion 70 and a back face of the seat flange 73.
The annular flow portion 74 includes a rear and a forward cylindrical flow segment that are joined by a forwardly inclined segment, the rear and inclined segments being provided on an outer circumferential and forward surface of the seat flange 73 respectively. The annular flow portion 74 cooperates, in use, with a complementary annular flow portion 74′ defined along the inside of the ring member 64 for defining an annular melt passageway 77 therebetween when the ring member is in the recovery position.
The tip flange 81 arranged along the remaining length of the tip member 62 provides the forward retaining portion 76, the tip circumferential portion 80, the tip tapered portion 82, and the tip planar portion 86, on a back face, outer face, forwardly inclined face, and front face, respectively, thereof. Four radially-spaced axial melt discharge grooves 84 are also arranged equiangular-spaced around the tip flange 81 that extend between the back and front faces and through the outer and inclined faces of the tip flange 81, have a longitudinal axis that is parallel to that of the tip member 62, and have about the same depth as the forward segment of the annular flow portion 74. The melt discharge grooves 84 provide passageways for the discharge of the melt from the annular melt passageway 77 into the accumulation region at the front of the barrel bore 48A.
The forward retaining portion 76 cooperates, in use, with a complementary forward retaining portion 76′ defined on the ring member 64 for limiting the forward travel of the ring member 64. When the forward retaining portion 76′ of the ring member 64 is forced to engage the complementary forward retaining portion 76 of the tip member 62, under the influence of the melt pressure being generated behind the non-return valve 60 by the rotation of the screw 56, the ring member 64 is in its recovery position and the annular melt passageway 77 is open. The surface of the forward retaining portions 76, 76′ are shown as being provided by on a resilient hard-facing material 78 in view of avoiding deformation thereof from the cyclic impacts of the forward retaining portions 76, 76′.
The diameter of the tip circumferential portion 80 is configured to cooperate with the barrel bore 48A to assist in the alignment of the tip member 62 with barrel bore 48A, this alignment is otherwise provided by a close fit between the screw flights 58 within the barrel bore 48A which keeps the screw 56 generally aligned therewith (for practical reasons there is typically enough of a gap between the screw flights 58 and the barrel bore 48A such that the screw 56 is never in precise alignment therewith as it sags under its own weight). During injection, the tapered portion 82 and the planar portion 86 both function to pressurize the melt in front thereof as the non-return valve 60 is forced forward along the barrel bore 48A.
The ring member 64 comprises an annular body 94. An outer circumferential surface 96 of the annular body 94 is configured to fit closely within the barrel bore 48A and to cooperate therewith to guide the ring member 64 as it is forced to translate therealong during recovery and injection. The ring member 64 and the tip member 62 remain generally mutually aligned, as characterized hereinbefore, given their centering using the barrel bore 48A, however there is no direct means provided for alignment therebetween.
The ring member 64 also includes a piston ring seat 95 formed as a circumferential groove through the outer circumferential surface 96 between the ends of the ring member 64. The piston ring seat 95 is configured to receive a piston ring 98 and a sub-ring 100 therebeneath, an outer surface 99 of the piston ring 98 provides a seal between the ring member 64 and the barrel bore 48A to prevent the bypass of melt therebetween during injection. A plurality of pressure ports 97 are provided that connect the annular melt passageway 77 with the piston ring seat 95 and which function to pressurize a region behind the sub-ring 100 with melt, during injection. The pressurization of the region behind the sub-ring 100 causes the sub-ring 100 and the piston ring 98 to radially expand which imparts a sealing force between the outer surface 99 of the piston ring 98 and the surface of the barrel bore 48A. The sub-ring 100 operates to seal a split provided in the piston ring and thereby limit the escape of the pressurizing melt. The annular flow portion 74′ is configured to follow the profile of the complementary annular flow portion 74 of the tip member 62, in a spaced arrangement therewith, so as to form the annular melt passageway 77 therebetween.
A rear retaining-sealing portion 90′ is configured along a rear inwardly tapered face of the annular body 94. The rear retaining-sealing portion 90′ functions to cooperate with a complementary rear retaining-sealing portion 90, provided on a front face of the flange member 66, to both limit the backward travel of the ring member 64 and to provide a face-seal 91 therebetween, when the ring member 64 is forced into the injection position, that is intended to prevent the backflow of melt during injection.
Similarly, a forward retaining portion 76′, the function of which was described hereinbefore, is provided on a planar front face of the annular body 94. The front retaining portion 76′ is shown as being provided on a hard-facing material 78.
The flange member 66 comprises an annular body 92 with a spacing flange portion 93 projecting from the base of a tapered front face thereof. An inner circumferential surface extends across the annular body 92 and provides a complementary aligning portion 70′ that is configured to cooperate with the aligning portion 70 of the tip member 62 for aligning, as described hereinbefore, the flange member 66 on the tip member 62. A rear planar face of the annular body 92 provides a complementary mating face 57′ that is configured to cooperate with the screw mating face 57 in positioning a complementary seat portion 72′, provided on a front face of the spacing flange portion 93, into the seat portion 72, on tip member, thereby securely locating the flange member 66 therebetween. The front face of the annular body 92 provides the retaining-sealing portion 90, described hereinbefore, that is configured to cooperate with the complementary retaining-sealing portion 90′ of the ring member 64, in the injection position, to both limit the rearward travel of the ring member 64 and to seal therewith (i.e. close the melt passageway 77). The retaining-sealing portion 90′ is shown as being provided on a hard-facing material 78. The diameter of the flange member 66 is appreciably narrower than the barrel bore 48A such that an annular gap formed therebetween provides a melt passageway for the forward passage of the melt during recovery.
Another example of a commonly known non-return valve 60 for use in a barrel assembly 38 of an metal injection molding machine is shown with reference to FIGS. 4A, 4B, 4C & 4D. The structure and operation of this non-return valve 60 is principally the same as that previously described and shown in FIGS. 3A, 3B, & 3C. Accordingly, only the differences in structure and operation between the embodiments will be reviewed and the features that are common to both embodiments have been given similar reference numbers. Of notable difference are the following: the forward retaining portion 76′, on the ring member 64, has been sub-divided across a plurality of equiangularly-spaced standoffs 102; the ring member 64 includes two piston ring 98 installations around its outer surface, and that no sub-rings 100 are used therewith; and the general configuration of the tip flange 81. As can be seen with contrast to FIG. 3A, the tip flange 81 lacks the radially-spaced axial melt discharge grooves 84 as the tip circumferential portion 80 is configured to be much narrower than the barrel bore 48A and therefore the gap therebetween provides the discharge passageway. The tip circumferential portion 81 also includes a hexagonal arrangement of tooling flats 104 that assist in the installation and removal of the tip member 62 in the screw 56. Lastly, the tip flange 81 includes a ball-nose portion 87 in place of the planar portion 86, a change that is of no consequence to the pressurizing of the melt.
The proper operation of these types of non-return valve 60, during injection, is contingent on an intimate face-seal 91 provided between the complementary rear retaining-sealing portions 90′ and 90 that are arranged on the ring member 64 and the flange member 66 respectively. Without an intimate face-seal 91 significant leakage may occur in association with a sudden loss of the pressure drop across the ring member.
Although it isn't well understood, a failure to achieve and/or sustain an intimate face-seal 91 for the duration of the injection step is thought to be caused by factors including melt non-uniformity and the injection dynamics.
In further detail, the non-uniformity of the melt of molding material (i.e. the inclusions of un-melted and irregularly sized feedstock particles) may allow for solid feedstock to become trapped between the retaining-sealing portions 90, 90′.
In addition, the dynamics within the barrel assembly 38 during injection impart extreme and widely varying forces on the components of the non-return valve 60 to such an extent that it is quite often necessary to make undesirable concessions in an injection profile (i.e. speed and acceleration of the screw 56) to avoid unreliable valve closing (i.e. achieving and maintaining the face-seal 91), particularly during the transition between the steps of injection and packing wherein the non-return valve undergoes a near instantaneous deceleration. The extreme dynamics are the inevitable result of the extremely small amount of injection time available (i.e. the time required to completely fill the at least one molding cavity, that is typically in the range of 30-50 milliseconds) without otherwise encountering filling problems due to melt freezing, and due to the properties of the melt (i.e. low viscosity and compressibility). Consequently, there is an extremely abrupt screw 56 acceleration, high injection speed (typically upwards of 6 meters per second), and after the filling of the at least one molding cavity the lack of melt compressibility effectively stops the screw 56 instantaneously. The loss of intimate contact most often occurs at initial closing impact (i.e. the transition form recovery to injection), and at the end of injection stroke due to sudden deceleration of the screw.
Unfortunately, practical experience has also shown that the integrity of the retaining-sealing portions 90, 90, and hence the ability to achieve an intimate face-seal 91, degrades at an unacceptably fast rate under normal operating conditions. Frequent compensating adjustments to the molding process are therefore required to compensate for the increasing backflow across the non-return valve 60 (e.g. increasing the volume of melt in the accumulation region, decreasing the injection speed, etc). The rapid degradation of the retaining sealing portion 90, 90′ isn't well understood but contributing factors are thought to include: off-center loading thereon caused by poor alignment; high impact forces; and from trapped solids (e.g. un-melted feedstock, hard carbide particles which entered with the feedstock and that are a remnant from feedstock comminution, etc.). Furthermore, it has been noted that the higher the percent solids in the molding material the faster the retaining-sealing portions 90, 90′ degrade. To make matters worse still, the high melt temperature (typically upwards of 600° C.) may slowly anneal the substrate material of the valve components and thereby reduce the resiliency of the retaining-sealing surfaces 90, 90′.
The degradation of the retaining-sealing portions 90, 90 typically requires servicing or replacement of the non-return valve 60 in as few as 10-60,000 molding cycles (i.e. weeks of service) which is quite a time consuming and complicated undertaking given the extent to which the injection and barrel assemblies 29, 38 need to be disassembled and all of the routine complications caused by the solidified molding material remaining therein. This servicing requirement incurs large costs in lost productivity (i.e. low system availability) and need for highly skilled technical labor.
Hence, there is a need for an improved non-return valve for use in a molding system that is more durable and that provides reliable valve closing. In further detail, there is a need for a non-return valve for use in an injection molding machine for the molding of a metal alloy with an improved means for sealing against the backflow of molding material during injection.